1. Field of the Invention
The present invention relates to a waveguide photodiode, and more particularly to a buried waveguide photodiode used for optical communications systems or the like.
2. Background Art
Waveguide photodiodes, which must operate at high speed, have a structure in which the p/i/n junction portion is buried in a semi-insulative InP layer so as to reduce their capacitance. In the manufacture of a waveguide photodiode each layer is generally formed by use of the metal organic chemical vapor deposition method.
For example, on an InP substrate doped with Fe (iron) as an impurity, the method epitaxially grows, one at a time, an n-InGaAs contact layer, an n-InP cladding layer, an n-InGaAsP light confining layer, an i-InGaAs light absorption layer, a p-InGaAsP light confining layer, a p-InP cladding layer, and a p-InGaAs contact layer. Then, after predetermined areas of these layers are etched, an InP blocking layer doped with Fe (hereinafter referred to as a Fexe2x80x94InP blocking layer) is epitaxially grown on the n-InP cladding layer. After that, an insulating -film and p-type and n-type electrodes are formed.
The i-InGaAs light absorption layer is a layer with no impurity added thereto, or it is a low-concentration impurity layer of p-type or n-type. The i-InGaAs light absorption layer is sandwiched by the n-InGaAsP light confining layer and the p-InGaAsP-light confining layer, collectively forming a p/i/n junction.
Incidentally, the p-InGaAsP light confining layer, the p-InP cladding layer, and the p-type InGaAs contact layer are generally doped with Zn (zinc) as a p-type impurity. This has caused problems in that Zn diffuses from these layers to the i-InGaAs light absorption layer, turning it into a p-type layer.
A method for solving the problem of diffusion of the p-type dopant Zn is to replace Zn with Be (beryllium) (Japanese Laid-Open Patent Publication No. 9-64459, published 1997). This patent describes thermal diffusion of Zn spreading from a doped p-InP cladding layer and p-InGaAs contact layer to an undoped light absorption layer during epitaxial growth.
In the manufacture of a buried waveguide photodiode, after a p-InP cladding layer and a p-InGaAs contact layer are formed, a Fexe2x80x94InP blocking layer is formed. This has caused problems in that Zn diffuses into the Fexe2x80x94InP blocking layer due to the thermal history (heat) of the epitaxial growth process of the Fexe2x80x94InP blocking layer.
Furthermore, some of the Zn which has diffused into the i-InGaAs light absorption layer may further diffuse, depending on the thermal history.
If Zn diffuses in this way, not only does Zn reach as far as the undoped i-InGaAs light absorption layer and the Fexe2x80x94InP blocking layer, but also there will be a reduction in the concentration of Zn contained in the doped p-InGaAsP light confining layer, p-InP cladding layer, and p-type InGaAs contact layer. This has caused problems such as increased leakage current, deteriorated device characteristics, and increased operational voltage.
Generally the impurity concentration of the light absorption layer in waveguide photodiodes must be lower than that in electroabsorption modulators (EA modulators). However, none of the buried photodiodes employing a Fexe2x80x94InP blocking layer is known to include a light absorption layer whose impurity concentration is 1xc3x971016 cmxe2x88x923 or less.
The diffusion mechanism of Zn spreading into the Fexe2x80x94InP blocking layer is not thermal vibration, which causes Zn to diffuse into the undoped i-InGaAs light absorption layer. It is considered that the diffusion of Zn proceeds as Zn replaces the dopant Fe which is a phenomenon referred to as counter diffusion. A region in which the concentration of Zn has become equal to that of Fe (as a result of the replacement of Fe with Zn) is called a counter diffusion region, and the leakage current increases as the size of the counter diffusion region becomes larger. Any quantitative relationship between a counter diffusion region and characteristics of the device has not yet been reported. Furthermore, no dopant capable of reducing the counter diffusion region is yet known.
The present invention is made to solve the above problems, and its object is to provide a waveguide photodiode in which only a small amount of impurities diffuses into the light absorption layer and the blocking layer.
Another object of the present invention is to provide a waveguide photodiode having a small counter diffusion region.
Still another object of the present invention is to provide a waveguide photodiode having good device characteristics obtained as a result of reducing the parasitic capacitance.
Other objects and advantages of the present invention will become apparent from the following description.
According to one aspect of the present invention, a waveguide photodiode comprises a semiconductor substrate, an n-type cladding layer, an n-type light confining layer, an i-type light absorption layer, a p-type light confining layer, a p-type cladding layer, and a Fexe2x80x94InP blocking layer. The n-type cladding layer, the n-type light confining layer, the i-type light absorption layer the p-type light confining layer, and the p-type cladding layer are buried in the Fexe2x80x94InP blocking layer in that order over the semiconductor substrate. At least one of the p-type light confining layer and the p-type cladding layer contains as a p-type impurity a material selected from a group consisting of beryllium, magnesium, and carbon.
According to another aspect of the present invention, a waveguide photodiode comprises a semiconductor substrate, an n-type cladding layer, an n-type light confining layer, an i-type light absorption layer, a p-type light confining layer, a p-type cladding layer, a Fexe2x80x94InP blocking layer, and a p-type contact layer. Then type cladding layer, the n-type light confining layer, the i-type light absorption layer, the p-type light confining layer, and the p-type cladding layer are buried in the Fexe2x80x94InP blocking layer in that order over the semiconductor substrate. The p-type contact layer is formed on the p-type cladding layer. At least one of the p-type light confining layer, the p-type cladding layer, and the p-type contact layer contains as p-type impurities two or more materials selected from a group consisting of zinc, beryllium, magnesium and carbon.
According to another aspect of the present invention, a waveguide photodiode comprises a semiconductor substrate, an n-type cladding layer, an n-type light confining layer, an i-type light absorption layer, a p-type light confining layer, a p-type cladding layer, and a Fexe2x80x94InP blocking layer. The n-type cladding layer, the n-type light confining layer, the i-type light absorption layer, the p-type light confining layer, and the p-type cladding layer are buried in the Fexe2x80x94InP blocking layer in that order over the semiconductor substrate. In the Fexe2x80x94InP blocking layer, a width of a counter diffusion region of Fe and a p-type impurity is 0.5 xcexcm or less.
Other and further objects, features and advantages of the invention will appear more fully from the following description.